Prepare barbecue (medium-high heat). Whisk oil, vinegar, 1 tablespoon mint, mustard, and garlic in small bowl; season dressing to taste with salt and pepper. Brush lettuce and onion slices with dressing. Sprinkle with salt and pepper. Grill onion slices until charred in spots and softened, about 3 minutes per side. Transfer to platter; separate rings. Grill lettuce until charred in spots and slightly wilted, about 2 minutes per side. Transfer 2 wedges to each of 4 plates. Grill Halloumi until charred in spots and softened, about 2 minutes per side, depending on brand used. Scatter onion rings over lettuce. Divide Halloumi among plates. Drizzle with additional dressing, sprinkle with remaining 1 tablespoon mint, and serve immediately.